I own a minecraft server with my IRL friends using scalacube hosting. We using vanilla minecraft 1.16.5. As we were playing, we noticed that some of the spiders at night were spawning with swiftness. I have never seen this on any minecraft server I have hosted or a regular survival world. If I could have a brief explanation of what this could be, please let me know.
(Update) They are still spawning. There is no pluggins on this server or anything. We noticed that it was only spiders that were spawning with potion effects. As of the 1.6 update. They made it so they could spawn with potion effects. But this is too consistent, please leave an explanation in the comments if you have one.
This is a feature of the game. Spiders have a chance to spawn with potions effects. This was taken from the wiki:
On hard difficulty, spiders occasionally spawn with status effects. For each pack spawn, there is a (10×clamped regional difficulty)% chance of the game applying a status effect. This does not apply to cave spiders. If it decides to do so, then it selects the effect. The spider can spawn with following effects:
Speed (40% chance)
Strength (20% chance)
Regeneration (20% chance)
Invisibility (20% chance)